Recently, display devices using liquid crystal cells have been mounted in a variety of electronic devices, e.g. timepieces, telephones, and personal computers. Of such electronic devices, timepieces, cellular phones, etc. may be used at night or in darkness. Therefore, many of these electronic devices are equipped with a backlight unit for illuminating the liquid crystal cell. Backlight units used for this purpose can be divided into an aligned-light type in which a liquid crystal display panel and a light source are disposed to face each other across a light-diffusing plate, and an edge-light type in which a liquid crystal display panel and a light guide plate are disposed facing each other, and a light source is disposed adjacent to the light guide plate outside the peripheral edge thereof so that light from the light source enters the light guide plate through the peripheral edge surface thereof (for example, see Japanese Patent Application Publication No. 2002-42529).
The edge-light type backlight unit is more advantageous than the aligned-light type backlight unit in that it is possible to achieve a low profile and to minimize the influence of heat generated from the light source on the liquid crystal display panel.
FIGS. 5(a), 5(b) and 6 show a conventional edge-light type backlight unit. FIG. 5(a) is a plan view, and FIG. 5(b) is a sectional view taken along the line 5b-5b in FIG. 5(a). FIG. 6 is a schematic view showing the optical path of light emitted from a light source of the backlight unit.
As shown in the figures, the backlight unit has in a housing 1 a rectangular light guide plate 2 formed from a transparent plastic member, and four light-emitting diodes (hereinafter abbreviated as “LEDs”) 3 are provided as a light source adjacent to a peripheral edge surface 2a of the light guide plate 2 that serves as a light entrance surface. Light emitted from the LEDs 3 enters the light guide plate 2 through the peripheral edge surface 2a thereof and travels in the light guide plate 2 while undergoing total reflection therein. The bottom surface 2b of the light guide plate 2 is configured to have a multiplicity of prisms as shown in the figures so that, while the light entered into the light guide plate is undergoing total reflection between the top surface 2c and the bottom surface 2b of the light guide plate 2 during traveling, the incident angle of the light impinging on the top surface 2c, which is a light exit surface, gradually decreases, and the light eventually exits to the outside through the light exit surface. A part of the light emitted from the LEDs 3 exits to the outside through the bottom surface 2b of the light guide plate 2. The light, however, is reflected by a reflecting sheet 4, e.g. a silver- or white-color sheet, back to the light guide plate 2. The light exiting to the outside through the top surface 2c of the light guide plate 2 is directed toward a liquid crystal display panel (not shown) by a prism sheet 5.
It is of course desirable that the backlight unit has a capability to illuminate the back surface of a liquid crystal display panel with high intensity.
Further, for a liquid crystal display panel of, for example, a cellular phone, it is also desirable that the backlight unit illuminates the liquid crystal display panel so that the liquid crystal display panel has high brightness in particular when it is squarely seen from the front thereof.